Many ships suffer from engine overloading caused by heavy running propellers. A ship's propeller is heavy running when its size or pitch is improperly matched with the ship's engine, causing the engine to become overloaded thereby exceeding the manufacturer's limit for maximum continuous operation. The pitch of a propeller is the distance traveled by a vessel to which a propeller is attached when the propeller completes one revolution. The higher the pitch of a propeller, the more load is placed on the engine, and the lower the rpm will be. Conversely, the lower the pitch of the propeller, the lower the load placed on the engine, and the higher the rpm will be. Although mismatch of a propeller and engine can be present when a vessel is assembled, it more commonly develops gradually as an engine weakens due to age and wear. Problems associated with heavy running propellers can be serious, including increased maintenance costs and a necessary reduction in a ship's operating speed.
Several methods exist in the prior art for correcting heavy running propellers. For example, a heavy running propeller can be replaced with a new propeller designed to properly match the available power from the engine. While being an effective solution, the costs of such an operation, both from the new propeller, and from the labor and downtime of the ship incurred during replacement efforts, can be prohibitive.
Another technique for correcting a heavy running propeller consists of re-pitching the propeller without changing the camber of its blades. As illustrated in FIG. 1, this method involves the twisting of a propeller blade 104 relative to a hub 106 to which it is attached, from an original blade position 108 to a new blade position 110. Twisting the blade in this fashion decreases the pitch of the propeller by rotating a trailing edge 112 of the propeller blade 104 through an arc 116 towards the bow of the ship, while rotating a leading edge 118 through an arc 120 in a direction away from the bow of the ship. This method is advantageous over the above mentioned propeller replacement procedure because it avoids the expense of buying a new propeller. It has several shortcomings, however, since it requires the removal of the heavy running propeller and the use of a land-based propeller workshop with equipment large and powerful enough to twist the propeller blades relative to the hub. Additionally, all the while this lengthy procedure is being conducted, the ship must remain idle.
Still another means for correcting a heavy running propeller is the reduction of a propeller's blade areas. This can be done by reducing a propeller's diameter, or by reducing the trailing edge and/or leading edge of a blade by cutting away blade material and grinding the pressure side of the blade surface to slightly reduce the propeller's pitch and camber. In cases where the degree of overload is especially pronounced, it is sometimes necessary to combine diameter reduction with trailing edge cutting in order to achieve the desired diminution in loading. FIG. 2 illustrates a blade 202 which has undergone both diameter and trailing edge reduction. In FIG. 2, an area 204 has been removed from the trailing edge 206 of blade 202. In addition, an area 208 has been removed from a tip 210 of blade 202, decreasing the radius of the blade 202 by an amount 212.
Both trailing edge reduction and diameter reduction can be performed underwater, but both are time consuming, and normally require more time to be fully completed than an average port call of a ship to load or unload cargo. Thus either the ship must extend its stay, or the work must be performed during two or more successive stops. In addition, both trailing edge reduction and diameter reduction result in a decrease in propulsive efficiency, with diameter reduction having a more pronounced effect. Decreasing the blade section by cutting away portions of the leading edge also leaves the modified blade section more prone to cavitation. Further, the cutting and grinding involved in the process produces metal particles which can pollute the water in which the procedure is conducted. Moreover, both trailing edge reduction and diameter reduction are difficult to reverse, entailing costly and time-consuming welding to reattach previously removed blade areas.
Accordingly, there is a need for a cost effective technique for correcting a heavy running propeller which can be performed without removing the propeller from the ship—or removing material from the propeller's blades—and which can be completed in the time it takes a ship to load or unload cargo during a single port call.